A. M. M. Fonseca

Plasma confinement using biased electrode in the TCABR tokamak

Experimental data obtained on the TCABR tokamak (R = 0.61?m, a = 0.18?m) with an electrically polarized electrode, placed at r = 0.16?m, is reported in this paper. The experiment was performed with plasma current of 90?kA (q = 3.1) and hydrogen gas injection adjusted for keeping the electron density at 1.0 ? 1019?m?3 without bias. Time evolution and radial profiles of plasma parameters with and without bias were measured. The comparison of the profiles shows an increase of the central line-averaged density, up to a maximum factor of 2.6, while H? hydrogen spectral line intensity decreases and the C?III impurity stays on the same level. The analysis of temporal behaviour and radial profiles of plasma parameters indicates that the confined plasma enters the H-mode regime. The data analysis shows a?maximum enhanced energy confinement factor of 1.95, decaying to 1.5 at the maximum of the density, in comparison with predicted Neo?Alcator scaling law values. Indications of transient increase of the density gradient near the plasma edge were obtained with measurements of density profiles. Calculations of turbulence and transport at the Scrape-Off-Layer, using measured floating potentials and ion saturation currents, show a strong decrease in the power spectra and transport. Bifurcation was not observed and the decrease in the saturation current occurs in 50??s.

Suppression and excitation of MHD activity with an electrically polarized electrode at the TCABR tokamak plasma edge

Two reproducible regimes of tokamak operation, with excitation or suppression of MHD activity can be obtained using a voltage-biased electrode inside the edge of the TCABR tokamak. The experiment was carried out adjusting the tokamak parameters to obtain two types of discharges: with strong or weak MHD activity, without biasing in both cases. The plasma current was adjusted to cover a range of safety factor from 2.9 up to 3.5, so that when biasing was applied the magnetic island (3,1) could interact with the edge barrier. The application of biasing in subsequent discharges of each type resulted in excitation or suppression of the MHD activity. The results show that the dominant modes are m = 2, n = 1 and m = 3, n = 1 for excitation and partial suppression, respectively. In both regimes a strong decrease in the radial electric field is detected with destruction of the transport barrier and of the improved confinement caused by different mechanisms. The measurements include temporal behaviour of edge transport, turbulence, poloidal electric and magnetic fields, edge density, radial electric fields and radial profile of Hα line intensity. The explanation of the excitation and suppression processes is discussed in the paper.

Runaway discharges in TCABR

It is found in experiments carried out in Tokamak Chauffage Alfven Bresilien (TCABR) that two regimes of runaway discharges (RADs) with very different characteristics are possible. The RAD-I regime, which is similar to that observed in other tokamaks, can be obtained by a gradual transfer from a normal resistive to a RAD by decreasing the plasma density. This regime can be well understood using the Dreicer theory of runaway generation. The total toroidal current contains a substantial resistive component and the discharge retains some features of standard tokamak discharges. The second runaway regime, RAD-II, was recently discovered in the TCABR tokamak (Galvao R.M.O. et al 2001 Plasma Phys. Control. Fusion 43 1181). The RAD-II regime starts just from the beginning of the discharge, provided that certain initial conditions are fulfilled and, in this case, the runaway tail carries almost the full toroidal current. The background plasma is cold and detached from the limiter due to the recombination process. The primary Dreicer process is suppressed in the RAD-II and the secondary avalanche process dominates, even at the start-up phase, in the generation of the toroidal current. It is possible to trigger a transition from the RAD-I to the RAD-II regime using plasma cooling by gas puffing. The experimental results are shown to be in reasonable agreement with theoretical predictions based on the runaway avalanche process.

Alfvén wave heating and runaway discharges maintained by the avalanche effect in TCABR

Recent results of Alfven wave heating experiments and the characteristics of a new regime of runaway discharges found in Tokamak Chauffage Alfven Bresilien (TCABR) are discussed. (1) Wave excitation was carried out with one module of the antenna system, with and without a Faraday screen. Evidence of plasma heating was obtained in both cases, for coupled wave powers up to half of the Ohmic power, approximately, without uncontrollable density rise during the RF pulse. The antenna coupling with the plasma seems to have increased when the Faraday screen was removed. (2) The new regime of runaway discharges is produced by initiating the main plasma breakdown without pre-ionization and strongly increasing the neutral gas fuelling at the end of the current ramp-up phase. Consequently, the plasma cools down substantially and switches to a runaway mode in conditions under which the primary (Dreicer) mechanism is strongly suppressed. This new regime of runaway discharges is characterized by strong enhancement of the relaxation oscillations, which are seen in the H α and ECE emissions, coupled with large spikes in the line density, loop voltage, bolometer, and other diagnostic signals.

ECE radiometry in the TCABR tokamak

A millimeter/microwave detection system, in operation in the TCABR Tokamak is described. The system is used for electron cyclotron measurements. The main part of the system is a heterodyne sweeping radiometer based on a BWO oscillator that operates in the frequency range of 52 to 85GHz. The system operates in two modes : fixed frequency (maximum resolution of 10 ms) and sweeping mode (50 ms per frequency step). The radiometer is calibrated in frequency and in radiation intensity. The frequency calibration is made by means of a precision harmonic oscillator. The absolute calibration was done using a blackbody (microwave absorber) immersed in liquid nitrogen (77 K) and also put in an oven with adjustable temperature up to 1470 K. Two others components are also used for periodic intensity calibration check and sensibility measurements : a Criogenic Matched Load and a Noise Source. A Gaussian antenna is used for better space resolution measurements. Between the antenna and the radiometer, oversized waveguides are used to reduced the signal attenuation. The antenna axis is in the equatorial plane of the machine and perpendicular to the plasma column axis. The accessibility and absorption conditions are discussed. Results showing time and radial profi les of the detected ECE radiation for the TCABR are presented. For a magnetic fi eld of BTO = 1.14T it was verifi ed that the maximum permissible density to access the second harmonic in the X mode is ne0 @ 2.3 × 1019 m-3.

Electron density measurements from right-hand cutoff of ECE in the TCABR tokamak

In tokamak machines with low toroidal magnetic fields and high plasma densities, the accessibility conditions impose restrictions to the detection of the Electron Cyclotron Emission (ECE). In these machines, the righthand cutoff condition can be used as an independent method to determine the local electron density from the ECE data in thermal discharges. In this paper is shown the results obatined from the detection of ECE radiation in the TCABR tokamak, in operation at the Institute of Physics of University of Sao Paulo. The effect of the ECE radiation cutoff was observed for different radial positions of the plasma column. To reach the ECE cutoff condition, the electron density was increased monotonically by the use of an external gas puffing system. For sufficient high densities, the emission at some frequencies is cutoff and the first and the last frequency to be cutoff depends on the shape of the density profile. These measurements do not require the plasma to be optically thick. It was observed that, for a toroidal field BO = 1:14T, the first cutoff of ECE occurs for a radial position r @ 5 cm. From these measurements the radial electron density was determined. For a symmetric parabolic profile ne = neo [1 - (r/a)2]a , values of a between 0.86 and 0.97 were experimentally obtained. A good agreement of these values with those obtained from the microwave interferometer measurements (a »0:85) was found. Therefore, the ECE right-hand cutoff constitutes an independent method to obtain information about the electron density profile.

Overview of Recent Results of TCABR

An overview of recent results obtained in TCABR is presented. Experiments on Alfven wave heating have been carried out in both low and high density regimes. Controlling the density rise usually observed in Alfven heating experiments, it was possible to get a clear confirmation of electron temperature increase in low‐density discharges. In the high density regime, the Alfven wave absorption occurs at mode numbers quite different from those for low density. Detailed experiments have been carried out on the transition between low and high‐density confinement regimes, triggered by electrostatic polarization at the plasma edge. The results indicate that the flatness of the density profile and the decrease of edge recycling depend strongly on the level of MHD activity during transition. A preliminary analysis of the electromagnetic emission associated with the relaxation instability in the new regime of runaway discharges discovered in TCABR shows that the observations are coherent with theoretical models. The heat transport in the presence of large magnetic islands has been investigated, in the collisional regime, and found to be properly described by the Fitzpatrick model. Finally, two diagnostic techniques have been further improved, the determination of the position of the local Alfven resonance by microwave reflectometry and the determination of the temperature and density at the plasma edge by the method based upon the uniqueness of the particle confinement time, determined from the hydrogen Balmer series emission.

Not completely flattened radial profile of the electron temperature in the vicinity of magnetic islands in Tokamak Chauffage Alfvén Brésilien

Recent results obtained in TCABR (Tokamak Chauffage Alfven Bresilien) [J. H. F. Severo, I. C. Nascimento, V. S. Tsypin, and R. M. O. Galvao, Nucl. Fusion 43, 1047 (2003)] show a nonmonotonic variation of the poloidal rotation velocity at the position of major magnetic islands. In this paper, the associated effect of the magnetic islands on the radial profile of the electron temperature is discussed. Analytical temperature profiles are used to analyze the experimental data obtained with electron cyclotron emission radiometry. It is shown that the competition between strong anomalous perpendicular diffusive transport and parallel heat convection is the dominant mechanism for the oscillations observed in the radial profile of the electron temperature in TCABR.

Recombinative plasma in electron runaway discharge

Cold recombinative plasma is the basic feature of the new regime of runaway discharges recently discovered in the Tokamak Chauffage Alfven Bresilien tokamak [R. M. O. Galvao et al., Plasma Phys. Controlled Fusion 43, 1181 (2001)]. With low plasma temperature, the resistive plasma current and primary Dreicer process of runaway generation are strongly suppressed at the stationary phase of the discharge. In this case, the runaway avalanche, which has been recently recognized as a novel important mechanism for runaway electron generation in large tokamaks, such as International Thermonuclear Experimental Reactor, during disruptions, and for electric breakdown in matter, is the only mechanism responsible for toroidal current generation and can be easily observed. The measurement of plasma temperature by the usual methods is a difficult task in fully runaway discharges. In the present work, various indirect evidences for low-temperature recombinative plasma are presented. The direct observation of recombinative...

Application of microwave reflectometry to register Alfvén wave resonances in the TCABR tokamak

Application of microwave reflectometry to study Alfven wave resonances in the TCABR tokamak is described. A microwave reflectometer was used to register plasma density oscillations driven by the excited Alfven waves, under the condition of the spectrum scanned by a controlled plasma density rise. It is shown that when the position of the local Alfven resonance rA, which is defined by the relation ω=k∥(rA)CA(rA), is close to the plasma zone where the microwave signal is reflected, the high-frequency modulation of the output signal of the reflectometer at the rf generator frequency increases. This method can give information about the localization of the rf power deposition zone in Alfven wave plasma heating and current drive experiments. It also allows finding the plasma current profile in the region of the rf power deposition.